DESCRIPTION: In the last decade, HIV incidence rates among young men who have sex with men (YMSM) in U.S. urban centers have steadily increased. To shed light on these trends, since 2009, we have been conducting a cohort study of n = 600 racially/ethnically and socioeconomically diverse YMSM, informed by a theory of syndemics, to: (1) develop and test theoretically informed measurement models of the covariance of illicit drug use, unprotected sexual behavior, and mental health burden among emergent adult HIV-negative YMSM within and across time; (2) assess whether patterns of behavior are continuous, discontinuous, or some combination of both; (3) delineate the risk and protective base (physical, relational, and psychosocial factors) that predict the development of syndemics: and (4) determine the extent to which the development of syndemics is moderated by race/ethnicity, social class, and homelessness/housing stability. At baseline, participants were 18-19 years old and are returning for semi-annual follow-up visits; to date we have high levels of participant retention (e83%) across study visits. Presently, we seek to develop a more comprehensive understanding of the health of this new generation of YMSM. This will be accomplished by (a) continuing to investigate the original study aims as participants mature to ages 25-26; and (b) including two new, complementary study aims to: (1) describe the social and sexual networks of YMSM, and examine the relationship between social and sexual network-level structural characteristics, social support and normative influences on syndemic production in YMSM, singly and in combination with physical, psychosocial, and relational predictors, both within and across time; and (2) describe the acquisition of sexually transmitted infections (STIs), specifically, gonorrhea chlamydia, and syphilis, among YMSM within and across time, and to determine whether physical, relational, and psychosocial factors explain STI acquisition within and across time. We will continue to test syndemic production over time as well as to examine how this comprehensive range of physical, psychosocial, and relational predictors are related to syndemic production using additive and multiplicative (synergistic) models. Next, for YMSM in this study who have seroconverted as well as those who may seroconvert, we include an exploratory aim to describe HIV clinical treatment markers (i.e., HIV viral load, ART uptake and adherence, HIV care) and to assess the extent to which physical, relational, and psychosocial factors are associated with these clinical markers, both within and across time. We seek to extend our cohort study through the addition of semi- annual data collection for 7 additional waves of data collection. We will recruit an additional n = 329 to add to our extent study sample. New recruits will be 21-22 years old at enrollment, and with our original n=471 active and available YMSM participants, will yield a sample of n = 800 at the first assessment of our competing continuation study. As with our current study we will utilize complex modeling (Structural Equation and Latent Growth Curve Analysis and Survival Analysis) to answer our study questions.